Guillermo del Toro has reportedly been forced to post-convert his upcoming blockbuster Pacific Rim into 3D by Warner Bros.

The Pan's Labyrinth director explained in July that he didn't want to utilise the format for the action movie, which sees Charlie Hunnam and Idris Elba fight off an alien invasion by piloting giant robots.

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"I didn't want to make the movie 3D because when you have things that big, the thing that happens naturally is you're looking at two buildings at 300 feet," he told Collider at Comic-Con.

"They're so big that you barely notice anything no matter how fast you're moving, so to force the 3D effect on robots and monsters that are supposed to be that high, you're making them miniaturised, you're making them human-scale.

"I knew that the 3D effect sounded like a great idea, but it was gonna be counter."

Warner Bros, who are releasing the film next summer, apparently "strong-armed" del Toro into the 3D conversion, according to Variety.

The uptick in box office receipts from 3D ticket sales is likely to be one of the reasons the studio has forced the conversion with del Toro.

It will also mean that Pacific Rim can be released in the lucrative Chinese market, which only shows US movies if they're screened in 3D or IMAX.

Pacific Rim's 3D conversion is expected to be completed in time for its July 12 release next year.